Inorganic Chemistry, Vol. 18, No.1 , 1979 51
Molecular Addition Compounds
(6) M. G. B. Drew, Prog. Inorg. Chem., 23, 67 (1977). (7) R. 0. Day, private communication.
References and Notes (1) C. J. Donahueand R. D. Archer, J . Am. Chem. SOC.,99,6613 (1977). (2) G. J. J. Chen, R. 0. Yelton, and J. W. McDonald, Inorg. Chim. Acta, 22, 249 (1977). (3) F. E. Mabbs and D. J. Machin, "Magnetism and Transition Metal
Complexes", Chapman and Hall, London, 1973. (4) R. Hoffmann, B. F. Beier, E. L. Muetterties, and A. R. Rossi, Inorg. Chem., 16, 511 (1977). (5) M. W. Anker, R. Colton, and I. B. Tomkins, Aust, J. Chem.,20,9 (1967).
(8) L. D. Brown! S. Datta, J. K. Kouba, L. K. Smith, and S. S. Wreford, Inorg. Chem., 17, 729 (1978), and references therein: (9) M. W. Anker, R. Colton, and I. B. Tomkins, Aust. J . Chem., 21. !159 (1968). (10) A. Mawby and G. E. Pringle, J . Inorg. Nucl. Chem., 34, 517 (1972). (11) M. G. B. Drew, J . Chem. SOC.,Dalton Trans., 626 (1972). (12) M. G. B. Drew, A. W. Johans, A. P. Wolters, and I. B. Tomkins, Chem. Commun., 819 (1971).
Contribution from the Richard B. Wetherill Laboratory, Purdue University, West Lafayette, Indiana 47907
Molecular Addition Compounds. 5. Interaction of N,N,N',N'-Tetramethylethylenediamine with Boron Trifluoride and Monoalkylboranes H E R B E R T C. BROWN,* B A K T H A N S I N G A R A M , and J O H N R . S C H W I E R
Receioed September 7 , 1978
N,N,N',N'-Tetramethylethylenediamine (TMED) reacts instantly with boron trifluoride ethyl etherate to give the highly insoluble product TMED.2BF3. This development made it of interest to synthesize and study the addition compounds of T M E D with representative monoalkylboranes. T M E D reacts readily with typical monoalkylboranes to form both the TMED-monoalkylborane TMED-BH2Rand the corresponding bis adducts TMED-2BH2R. These are air stable and can be stored without apparent change for long periods of time. Boron trifluoride removes T M E D from these adducts, rapidly and completely, precipitating TMEDeZBF,. In this way the monoalkylboranes can be conveniently purified and stored a s their stable adducts with T M E D , with rapid convenient generation of the parent monoalkylborane as desired.
Introduction Recent studies in this laboratory have resulted in a simple procedure for the preparation of triethylamine-monoalkylborane adducts, Et3N.BH2R, starting from triethylaminethexylborane, Et,N-BH,Th (l),l and have led to the discovery of promising new applications for these derivatives. The triethylamine component could be removed from these adducts by treatment with either T H F s B H or ~ ~Et20.BF3, to produce the free monoalkylboranes for reduction and hydroboration applications. However, this procedure suffers from certain difficulties. Both Et3N.BH3 and Et3N.BF3 are highly soluble in the usual THF medium, making them difficult to separate from the desired p r ~ d u c t . ~This . ~ problem can be circumvented in part by changing to a pentane solution from which Et3N-BF3 can be precipitated at -50 O C 3 Furthermore, the Et3N.BH2R adducts are liquids of uncertain purity which cannot be purified readily. Moreover, the versatile monoalkylboranes, thexylborane, ThBH2,4and 2,4,4-trimethyl-3-pentylborane, diisobutylborane (DIB)BH2,5 possess limited stability upon storage in THF at 0 "C or at 25 "C5Consequently they must be synthesized and used shortly thereafter. Hence it appeared highly desirable to develop a derivative which could be stored either neat or in solution for extended periods of time and then conveniently converted to the free borane as and when needed. In the course of our work on amine-boranes: we discovered that the reaction of N,N,N',N'-tetramethylethylenediamine (TMED) with Et,0.BF3 affords a white solid which is highly insoluble in the usual organic solvents (THF, E t 2 0 , CHC13, pentane, benzene) and only slightly soluble in acetone or water. Despite the fact that many studies have been published concerning addition compounds of polyamines with boron trifluoride, no results on the reaction between Et20.BF, and T M E D appear to have been recorded in the literature prev i ~ u s l y . ~ -Therefore, '~ it appeared desirable to investigate the reaction between Et20.BF, and T M E D and to explore the possibility of utilizing T M E D as a stabilizing addendum for monoalkylboranes. 0020-1669/79/1318-0051$01.00/0
This paper reports the isolation and characterization of T M E D adducts of BF,, ThBH2, (DIB)BH2, and monoisopinocampheylborane, (IPC)BH2, and the facile quantitative removal of the T M E D complexing agent from these adducts as the highly insoluble TMEDsZBF, compound. Experimental Section The reaction flasks and other glass equipment used for experiments were dried in an oven and assembled in a stream of dry nitrogen gas. Special experimental techniques used in handling air-sensitive material are described in detail e1~ewhere.l~ All melting points are uncorrected and were determined in evacuated sealed capillary tubes using the Thomas-Hoover capillary melting point apparatus. Et,O.BF, and T M E D were distilled from calcium hydride. 'H N M R and I1B N M R spectra were recorded on Varian T-60 and FT-80A instruments, respectively. The IH and IlB chemical shifts are in 6 relative to Me& and Et,O.BF, standards, respectively. Preparation of Complexes.15 N,N,N',N'-Tetramethylethylenediamine Complex with Boron Trifluoride,TMEDaZBF, (2). (a) Determination of Stoichiometry by 'HNMR Spectroscopy. Three different reactions were carried out in individual reaction flasks maintained at 25 "C. The flasks were charged with TMED (2.0 mmol), benzene (2.0 mmol, internal standard), and CDC1, (2.0 mL). An aliquot was taken from the first flask and the amount of T M E D was estimated via ' H N M R spectroscopy. To the second flask, 2.0 mmol of Et,O.BF, was added with stirring. The 'H N M R spectrum of the supernatant liquid indicated that 1.O mmol of T M E D remained unreacted. To the third flask was added with vigorous stirring 4.0 mmol of Et,O-BF,. The supernatant liquid was withdrawn and its IH N M R spectrum determined. N o signals attributable to T M E D were visible. (b) Determination of Stoichiometry by GLC. The same reactions were carried out in the T H F , Et,O, and pentane at 25 OC. The following procedure in T H F is representative. In a 50-mL centrifuge vial 5.0 mmol of T M E D and 3.0 mmol of n-dodecane (internal standard) were dissolved in 4.0 mL of T H F . The amount of T M E D present in the solution was determined by G L C analysis with a 6 ft X 0.25 in. column packed with 10%Carbowax 20M on Chromosorb W . The solution was then treated with 5.0 mmol of Et,O.BF, with stirring; a white solid precipitated. G L C analysis of the supernatant liquid indicated that 2.5 mnjol of the T M E D remained unreacted. The reaction mixture was treated further with 5.0 mmol of Et20.BF3.
0 1979 American Chemical Society
52 Inorganic Chemistry, Vol. 18, No. 1, 1979
Brown, Singaram, and Schwier
The G L C analysis now revealed that none of the T M E D was present in the solution. The adduct was collected by centrifugation, washed several times with n-pentane, and dried. There was obtained 1.254 g (99.5% yield): mp 210-212 OC (recrystallized from acetonitrile); ' H N M R (acetone-d,) 6 2.97 (s, 12 H ) , 3.53 (s, 4 H ) ; "B N M R 6 -0.74 (s). The same adduct precipitated when Et20.BF3 was added to excess T M E D . The results were identical with reverse addition. TMED.BH2(DIB) (3). In a 50-mL centrifuge vial 10.0 mmol of borane-methyl sulfide (BMS) was placed. To this reagent 1.57 mL of 2,4,4-trimethyl-2-pentane (10.0 mmol) was added dropwise with stirring. Stirring was continued for another 0.5 h at 25 " C . To the reaction product 1.5 m L of T M E D (10.0 mmol) was added slowly. The solid adduct was washed with cold n-pentane to remove methyl sulfide and then dried. There was obtained 2.39 g (98.7% yield) of the 1 : l adduct: m p 92-95 " C (recrystallized from n-pentane); ' H N M R (CDCI3) 6 0.12 (small hump, 1 H), 0.95-1.05 (s and a, 15 H), 1.83 (m, 1 H ) , 2.22 (s, 6 H ) , 2.50 (s, 6 H ) , 2.60-3.20 (m, 4 H); "B N M R 6 -1.74 (br t). TMED.BH,Th (4). Thexylborane (10.0 mmol) with methyl sulfide was prepared according to the literature procedure.16 To this product 1.5 mL of T M E D (10 mmol) was added a t 25 " C with stirring. Following completion of the vigorous reaction, methyl sulfide was removed under reduced pressure (12 mmHg) to afford 1.98 g of TMED.BH2Th as a viscous liquid (92% yield). The compound was purified by distillation under reduced pressure: bp 38-40 OC (12 mmHg); nZ0D 1.4240; IH N M R (CDCI?) 6 0.83-0.93 (s and d, 12 H), 1.08-1.60 (m, 1 H ) , 2.25 (s, 6 H), 2.65 (s, 6 H ) , 2.65-3.00 (m, 4 H); "B N M R 6 --1.38 (t). TMED.2BH2Th (5). The procedure is the same as above, except that only 5 mmol of T M E D (0.75 mL), instead of 10 mmol, was used to form the addition compound. Removal of methyl sulfide under reduced pressure (12 mmHg) provided the adduct as a semisolid: 1.46 g (93% yield). i t was then purified by recrystallization from n-pentane a t -50 "C: nip 43-45 OC; ' H N M R (CDCIJ 6 0.83 (s, 12 H ) , 0.87 (d, J = 6 Hz, 12 H ) , 1.47 (septet, J = 6 Hz, 2 H), 2.63 (s, 12 H ) , 3.2 ( s , 4 H); "B N M R 6 -0.77. TMED.BH,(IPC) (6). With the usual experimental setup, neat Et,N-BH,(IPC) (5 mmol) was prepared as described earlier.' To this reagent 0.75 mL of T M E D (5.0 mmol) was added at 25 " C and the mixture was stirred for 1.5 h. The heavy white precipitate of TMED.BH,(IPC) was collected by centrifugation, washed with cold n-pentane, and dried. There was obtained 1.01 g (75% yield) of the 1:l adduct: mp 113-1 15 "C (recrystallized from n-pentane); IH N M R (CDC13) 6 1.00 (d, J = 6 Hz, 3 H ) , 1.10 ( s , 3 H ) , 1.17 (s, 3 H), 2.27 (s, 6 H ) , 2.60 (s, 6 H ) , 3.2 (s, 2 H ) ; "B N M R 6 +2.20. TMED-2BH2(IPC)(7). A solution of TMED.BH,(IPC) (10 mmol) in E t 2 0 (10 mL) was taken in a 50-mL centrifuge vial and 1.23 m L of Et20.BF, (10 mmol) was added to it with constant stirring. On completion of the addition, the solid TMED-2BF3 was centrifuged and the supernatant liquid was transferred to another centrifuge tube and cooled. The crystalline adduct was collected in the usual manner and dried. There was obtained 1.69 g of the 1:2 adduct (80% yield): m p 140-141 " C (recrystallized from Et20); 'H N M R (CDCIJ 6 1.0 (d, J = 6 H z , 6 H), 1.1 (s, 6 H), 1.16 (s, 6 H ) , 2.63 (s, 12 H), 3.2 (s, 4 H); "B N M R 6 +1.80. Liberation of Free Monoalkylborane. The following procedure for the liberation of free (DIB)BH2 from TMED.BH2(DIB) in T H F is representative. To a solution of 10 mmol of TMED.BH2(DIB) in I O mL of T H F was added 20 mmol of Et20.BF3with stirring at 25 "C. A heavy white solid precipitated almost immediately. Stirring was continued for 15 min. The mixture was diluted with 10 mL of T H F and the solid centrifuged down. The supernatant solution was decanted. Examination by 1R revealed that it contained essentially 10 mmol of free (DIB)BH2.
Results and Discussion Our primary interest was to establish the stoichiometry of the reaction between Et,O.BF, and T M E D and the nature of the solid obtained. The reaction proceeds simply to the formation of 1.2 complex (eq l ) , with no evidence for the TMED
+ 2EtzO.BF3
THF
TMED*2BF,i
(1)
formation of the intermediate 1 :1 complex. The course of the reaction could be followed conveniently by 'H N M R spec-
troscopy and GLC. Methyl protons in T M E D resonate a t 6 2.25, and by integration of this peak the amount of unreacted amine could be calculated using an internal standard. Similarly the amount of uncomplexed amine could be determined also through GLC analysis. The solid is readily isolated. Upon recrystallization from acetonitrile large crystals are obtained. Elemental analysis revealed a molecular formula C ~ H I ~ B for ~ Nthe~ adduct. F ~ 'H N M K spectroscopy indicated that the adduct could be represented either as the symmetrical bis adduct (2) or as the boronium tetrafluoroborate (8). H3C C H 3
'
CH2-k'
CH3
CH:,
1
F3B:N-CH2-CH2--N:BF3
1
I
\\
1
t
BF2
BF4-
I
8
Since structure 2 is highly symmetrical, its IlB N M K spectrum will have a single resonance line. On the other hand, the I'B N M R spectrum of a compound represented by structure 8 would consist of two boron resonance lines. '4ctually the spectrum of the adduct displayed a single boron resonance line, thereby supporting structure 2 for the adduct. To the best of our knowledge, TMED.2BF3 has not been previously reported in the literature. In this way, Et,O.BF, can be utilized to remove TMED quantitatively from solution in a wide variety of solvents. Alternatively, T M E D can be used to precipitate BF3 quantitatively from solutions in these solvents. Any excess T M E D over the 1:2 stoichiometry remains unreacted. Thus, the addition of Et20.BF3 to an equimolar amount of T M E D in THF precipitates precisely half of the amine as the adduct with half of the amine remaining in solution (eq 2). Identical results were obtained in Et20 and TMED
+ EtzO.BF3
THF __*
25 " C
'/ZTMED*2BF3J
+ '/,TMED
(2) n-pentane. The same results were realized with reverse addition. This development made the T M E D adducts of monoalkylboranes of considerable interest. Accordingly, we turned our attention to the preparation and characterization of representative TMED-monoalkylborane adducts. TMEDBH,(DIB) and TMED.BH,Th were quantitatively prepared by direct reaction of (DIB)BH, and ThBH,, respectively, with T M E D in 1:l molar ratio (eq 3 and 4).
(DIB)BH,
+ TMED
ThBH,
+ TMED
THF
THF
TMED*BH,(DIB) (3) TMED-BHzTh
(4)
Since TMED can displace other amines from their adduct^,^ we utilized Et,N.BH,(IPC) for the preparation of TMED. BH,(IPC) (eq 5). Triethylamine was removed under reduced
6
$. E t 3 N
(5)
BH2'TMED
6
pressure (1 2 mmHg) to yield the 1: 1 adduct. The 1 :2 adducts could be prepared by combining 2 mol of the monoalkylborane
Inorganic Chemistry, Vol. 18, No.1, I979 53
Molecular Addition Compounds with 1 mol of TMED. In this way, TMEDa2BH2Th was prepared (eq 6). 2ThBH2
25 ‘C
+ TMED
neat
TMEDe2BH2Th
(6)
5
Treatment of the 1:l addition compounds with an equimolar quantity of Et,O.BF, precipitates half of the TMED and forms the 1:2 complex in solution (eq 7). This reaction was used 2TMEDsBHzR
+ 2EtzO.BF3
THF
2 5 DC, o,5 h *
TMEDs2BHzR
+ TMEDe2BF3J.
(7)
for the preparation of TMED.2BH2(IPC) (eq 8). 2TMED*BH,(IPC)
+ 2Et20.BF3
THF
25 oc, o,5
+ TMED.2BF34
(8) Both the 1:l and 1:2 addition compounds of these monoalkylboranes and T M E D are air stable and can be stored neat or in THF solution for several weeks a t 25 OC without noticeable hydride loss, isomerization, or redistribution. Treatment of the adducts in THF solution with 2 equiv of Et,0-BF3 rapidly regenerates the free monoalkylboranes in solution with the complete precipitation of TMED-2BF3 (eq 9) TMED-2BH2(IPC)
a
TMED-BHZR + 2EtzO.BF3
THF
25 oC, o,5 h *
RBH2
+ TMEDs2BF3J. (9)
Conclusion The fast, complete reaction of boron trifluoride with TMED to form the highly insoluble TMED.2BF3 provides a convenient
emove either T M E D or BF3 from solution. Monoalkylboranes are readily stabilized as their T M E D adducts, TMED.BH2R and TMED.2BH2R. Treatment of appropriate solutions of these adducts with an equivalent quantity of Et20.BF3 rapidly precipitates TMED.2BF3 quantitatively, providing pure solution of the monoalkylboranes. This development makes such monoalkylboranes readily available for study and utilization. Acknowledgment. We are grateful to the National Institutes of Health (Grant GM 10937) for financial support. Registry No. 2, 67813-45-0; 3, 67826-88-4; 4, 67826-89-5; 5, 67826-90-8; 6 , 68297-73-4; 7, 68297-74-5; T M E D , 110-18-9; Et20.BF3, 109-63-7; BMS, 13292-87-0; 2,4,4-trimethyl-2-pentene, 107-40-4; Me2S.BH2Th, 68297-75-6; Et3N-BH2(IPC),64065- 16-3.
References and Notes (1) H. C. Brown, N. M. Yoon, and A. K. Mandal, J . Organomet. Chem., C10, 135 (1977). (2) H. C. Brown and N. M. Yoon, J . Am. Chem. Soc., 99, 5514 (1977). (3) H. C. Brown and A. K. Mandal, Synthesis, 2, 146 (1978). (4) E. Negishi and H. C. Brown, Synthesis, 2, 77 (1974). (5) Manuscript in progress with Dr. John R. Schuier. (6) S. U. Kulkarni and H. C. Brown, Inorg. Chem., 16, 3092 (1977). (7) J. M. Van Paasschen and R. A. Geanangel, Can. J . Chem., 53,723 (1975). (8) J. R. McDivitt and G.L. Humphrey, Spectrochim. Acta, Part A, 30a,
1021 (1974). (9) A. R. Gatti and T. Nortik, Inorg. Chern., 5, 2075 (1966). (10) A. R. Gatti and T. Nortik, Inorg. Chem., 5, 329 (1966). (1 1) R. A. Geanangel and S. G. Shore, “Preparative Inorganic Reactions”, Vol. 3, Wiley-Interscience, New York, N.Y., 1966, p 123. (12) L. T. Murray, Ph.D. Thesis, Purdue University, 1963. (1 3) H. C. Kelly and J. 0. Edwards, Inorg. Chem., 2, 226 ( 1 963). (14) H. C. Brown, G. W. Kramer, A. B. Levy, and M. M. Midland, “Organic Syntheses via Boranes”, Wiley-Intersciencr, New York, N.Y., 1975. (1 5) All compounds gave satisfactory elemental :%nalyses. (16) H. C. Brown, A. K. Mandal, and S. U. Kulkarni, J . Org. Chem., 42, 1392 (1977).
Contribution from the Richard B. Wetherill Laboratory, Purdue University, West Lafayette, Indiana 47907
Molecular Addition Compounds. 6. Addition Compounds of Ethylenediamine with Boron Trifluoride and Dialkylboranes’ H E R B E R T C. B R O W N * and BAKTHAN S I N G A R A M * Received September 7, 1978
Ethylenediamine (EDA) reacts readily with boron trifluoride in ether to give the insoluble mono adduct NH,CH2CH2NH2.BF3. The amine also reacts with dialkylboranes in a 1:2 molar ratio to provide the bis adducts EDA.2BHR2. The latter adducts are quite stable and can be stored at 0 “C without detectable isomerization or redistribution. In ether or THF, boron trifluoride readily and quantitatively removes EDA from these adducts liberating the free dialkylboranes. Thus this procedure provides a new valuable means for storing dialkylboranes as their stable EDA adducts, with rapid regeneration of the parent dialkylborane as desired.
Introduction One of the intriguing problems in borane chemistry has been the relative instability of many of the borane reagent^.^-^ Workers in this area have usually been acutely aware of the possibilities for isomerization and redistribution. Consequently, they have often been limited to the use of freshly prepared reagents and very mild reaction conditions. We recently observed the stabilization of monoalkylboranes using N,N,N’,N’-tetramethylethylenediamine (TMED). In the hope of achieving a similar stabilization of dialkylboranes, we extended our study of the possible stabilization of these derivatives as addition compounds. For example, dicyclohexylborane ((cHx),BH), disiamylborane (Sia2BH), and diisopinocampheylborane ((IPC),BH) all possess limited stability upon storage. Anomalous results are obtained when hydroboration is carried out using an aged 0020-1669/79/ 1318-0053$01 .OO/O
sample of these dialkylboranes.6-8 Unfortunately, our attempts to stabilize these dialkylboranes with T M E D were unfruitful. The adducts appear to be dissociated, so that the dialkylboranes undergo the usual redistribution reactions. Presumably the dissociation is the result of conflicting steric requirements of the dialkylborane and the tertiary amine. Among less hindered diamines, ethylenediamine (EDA) appeared to be a promising candidate for a complexing agent for such dialkylboranes. The reaction between EDA and Et20.2BF3 is reported to afford either the mono or the bis adduct depending on the solvents used.9 The mono adduct has not been isolated in pure form and the structures of these adducts have nut been unambiguously e s t a b l i ~ h e d . ~ , ’ ~ This paper deals therefore with the isolation and characterization of the mono adduct of BF3 and EDA, the stabili-
0 1979 American Chemical Society
